As one who has made a career photographing from water, I know from costly experience there is no way to be "totally waterproof". But, you can vastly improve your odds if you take a few precautions.
The hard plastic case (Pelican or similar) is one of the best solutions, but, as pointed out, is probably too bulky to be useful when kayaking. Same goes for what I usually use, a large cooler, in which I fit two or three bodies and a few long lenses.
I also kayak quite a bit and, when I do, I use a dry bag. While it would be smaller than a case, I'd make sure it was large enough that your equipment could be quickly taken out and stowed inside. (Not sure what is worse, missing that important shot because you didn't get to your camera fast enough or ruining it because you couldn't put it back before the big splash.)
Before you set out, I would make certain that your dry bag is truly dry. I'd run a test each time you use it, without any gear, just to make sure the all the seams hold and there are no pinhole leaks. It might have worked fine the last time, but setting the bag down on a rough surface, like rocks or gravel, even a small piece of class hiding in what you thought was a safe spot, can cause a puncture (likely one you can't see, but will still let water in). Also, avoid quick changes of temperature, as taking the bag from warm to cold, or even cold to warm, yields condensation.
Just to be extra safe, carry a towel in the bag as well, and use it to wipe up any moisture before it has a chance to find its way to your gear.
And, one more thing, make sure your bag is well-sealed, so give it at least three turns.